There has been known a liquid crystal display device having a liquid crystal cell such as a TN (Twisted Nematic) or STN (Super Twisted Nematic) liquid crystal cell which has a sandwich structure that nematic liquid crystals having positive dielectric anisotropy are sandwiched between substrates with a transparent electrode having a liquid crystal alignment film so that the long axes of liquid crystal molecules can be twisted at 0 to 360° continuously between the substrates as required or IPS (In Plane Switching) liquid crystal cell (refer to JP-A 56-91277 and JP-A 1-120528).
In this liquid crystal cell, in order to align the liquid crystal molecules in a predetermined direction with respect to the surface of the substrate, a liquid crystal alignment film must be formed on the surface of the substrate. This liquid crystal alignment film is usually formed by rubbing the surface of an organic film formed on the surface of the substrate with a cloth material such as rayon in one direction (rubbing method). However, since dust or static electricity is apt to be generated in the process when the liquid crystal alignment film is formed by rubbing, dust adheres to the surface of the alignment film, thereby causing a display failure. Particularly in the case of a substrate having a TFT (Thin Film Transistor) device, the circuit destruction of the TFT device is caused by the generated static electricity, thereby reducing the yield. Further, in a liquid crystal display device which will become fine and finer in the future, as the surface of the substrate becomes uneven along with the increasing density of pixels, uniform rubbing is becoming difficult.
As alternative means of aligning liquid crystals in the liquid crystal cell, there is known an optical aligning method for providing liquid crystal alignability by applying polarized or non-polarized radiation to a photosensitive thin film made of polyvinyl cinnamate, polyimide or azobenzene derivative formed on the surface of the substrate. According to this method, uniform liquid crystal alignment can be realized without generating static electricity and dust (refer to JP-A 6-287453, JP-A 10-251646, JP-A 11-2815, JP-A 11-152475, JP-A 2000-144136, JP-A 2000-319510, JP-A 2000-281724, JP-A 9-297313, JP-A 2003-307736, JP-A 2004-163646 and JP-A 2002-250924).
In a TN (Twisted Nematic) or STN (Super Twisted Nematic) liquid crystal cell, the liquid crystal alignment film must have a pretilt angle for aligning liquid crystal molecules at a predetermined angle with respect to the surface of the substrate. When the liquid crystal alignment film is to be formed by the optical aligning method, the pretilt angle is generally provided by inclining the direction of radiation applied to the surface of the substrate from the normal of the substrate.
As an operation mode of a liquid crystal display device different from the above liquid crystal display devices, there is known a homeotropic alignment mode for aligning liquid crystal molecules having negative dielectric anisotropy in a direction perpendicular to the substrate. In this operation mode, the liquid crystal molecules must be tilted from the normal direction of the substrate toward one direction within the plane of the substrate when the liquid crystal molecules are tilted in a direction parallel to the substrate by applying voltage between the substrates. As means for this, there are proposed a method in which projections are formed on the surface of the substrate, a method in which stripes are formed on a transparent electrode and a method in which liquid crystal molecules are slightly pretilted from the normal direction of the substrate toward one direction within the plane of the substrate by using a rubbed alignment film.
It is known that the above optical aligning method is useful for controlling the tilt direction of the liquid crystal molecules in the liquid crystal cell of a homeotropic alignment mode. That is, it is known that the inclination direction of the liquid crystal molecules at the time of applying voltage can be uniformly controlled by using a homeotropic alignment film provided with alignment control capability and pretilt angle generating capability by the optical aligning method (refer to JP-A 2003-307736, JP-A 2004-163646, JP-A 2004-83810, JP-A 9-211468 and JP-A 2003-114437).
The liquid crystal alignment film manufactured by the above optical aligning method can be effectively used in various types of liquid crystal display devices. However, the optical alignment film of the prior art has a problem that the dose of radiation required to obtain a large pretilt angle is large. For example, it is reported that 10,000 J/m2 or more of radiation whose optical axis is inclined from the normal of the substrate must be applied to obtain a sufficiently large pretilt angle when liquid crystal alignability is to be provided to a thin film containing an azobenzene derivative by the optical aligning method (refer to JP-A 2002-250924, JP-A 2004-83810 and J. of the SID 11/3, 2003, p. 579).